In recent years there has been a significant commercial interest in polymer blends which have a combination of both elastic and thermoplastic properties. These polymer blends have been given the generic designation of Thermoplastic Olefins ("TPO"). They exhibit some of the properties of a cured elastomer as well as the reprocessibility of thermoplastic resins. The elastomeric characteristics are enhanced if one component of the blend is a vulcanizable elastomer which is wholly or partially crosslinked.
The earliest work in the curing of TPO compositions area was by Gessler and Haslett; see U.S. Pat. No. 3,037,954. That patent teaches the concept of "dynamic curing" wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer cured while continuously mixing and shearing the polymer blend. The result is a micro-gel dispersion of cured rubber in an uncured matrix of resinous thermoplastic polymer Gessler, '954 discloses compositions comprising polypropylene and a rubber wherein the rubber can be, inter alia, butyl rubber, chlorinated butyl rubber, polybutadiene, polychloroprene and polyisobutene. Compositions of about 50 to 95 parts polypropylene and about 5 to 50 parts of rubber are disclosed.
U.S. Pat. Nos. 3,758,643 and 3,806,558 disclose TPO type polymers comprising an olefin resin and olefin copolymer rubber wherein the rubber is dynamically cured to a partial cure state. These compositions are reprocessible and result in molded products having good surface appearance. However, the potential applications of these products is limited because of high compression set and/or low softening temperatures resulting from only a partial cure of the rubber. Furthermore, the partial peroxide cure utilized is difficult to control from the standpoint of completeness of reaction, resulting in batch to batch variations in product properties.
U.S. Pat. No. 4,130,534 discloses TPO blends which comprise a polyolefin resin and a butyl rubber wherein the blend comprises up to 60 wt % of polyolefin resin and 40 wt % or more of a rubber. The butyl rubbers which can be utilized include butyl rubber per se as well as chlorinated or brominated butyl rubber. In the preferred embodiment, the polyolefin resin comprises 20 to 45 wt % of the blend and the rubber comprises about 80 to 55 wt % of the blend. The rubber is said to be fully cured, the cure being a dynamic vulcanization.
U.S. Pat. No. 4,130,535 discloses TPO compositions wherein the rubber component is an ethylene propylene copolymer ("EPM") or terpolymer ("EPDM"). The preferred thermoplastic polyolefin resins are polypropylene and polyethylene. The compositions comprise about 75 to about 25 wt % polyolefin resin and about 25 to about 75 wt % rubber. The rubber is dynamically cured to a fully cured state. Similar compositions are disclosed in U.S. Patent 4,311,628 wherein the cure system for the rubber is a metal activated haloresin cure, e.g., brominated dimethylol phenol resins.
These TPO systems wherein the rubber is fully cured have the disadvantage that, as a result of poor flow characteristics, injection molded components made from these TPO's exhibit "flow line" imperfections. As a consequence special mold designs are required to minimize the problem, especially for large parts. Applications are further limited because of the high Shore A hardness of the composition.
U.S. Pat. No. 4,409,365 discloses TPO compositions in which a polyolefin resin is combined with an EPDM and a nitrile rubber. The rubbers are vulcanized and the product is said to have improved oil resistance.
U.S. Pat. No. 3,081,279 discloses compositions wherein a rubber such as butyl rubber is combined with a sulfochlorinated polyolefin resin and cured. The uncured blend is said to have improved processing characteristics.
U.S. Pat. No. 2,369,471 disclosed blends of ethylene polymers with various hydrocarbon rubbers and halogenated rubbers. The compositions are either uncured or compression molded vulcanizates.
U.S. Pat. No. 4,302,557 discloses a shrink film which comprises an EPM or EPDM rubber and a low density polyethylene or soft ethylene copolymer such as ethylene vinylacetate Alternatively, the polymer blends can include polypropylene or high density polyethylene (HDPE). Another variant of the disclosed composition comprises the ethylene copolymer rubber with the polypropylene or HDPE The rubber can be unvulcanized or, in the alter: native a film of the blend is prepared and radiation cross-linked to form a heat shrinkable film.
U.S. Pat. No. 4,212,787 discloses a combination of a peroxide curable rubber and a peroxide decomposible polyolefin resin which may, alternatively have incorporated therein a non-peroxide curable rubber such as polyisobutylene. The peroxide curable rubber is preferably an ethylene propylene rubber (EPR) and the peroxide decomposible polymer is preferably polypropylene. The curable rubber is partially cured in the manner of U.S. Patent 3,866,558, supra. The partial curing can be carried out in the presence of divinyl benzene The product is said to have improved flexibility and rebound elasticity. The compositions comprise about 100 to 5 parts by weight of a partially cured elastomer which is prepared by the dynamic heat treatment in the presence of a peroxide of a mixture of 100 to 40 parts by weight of (a) peroxide-curable olefin copolymer rubber and 0 to 60 parts by weight of (b) a peroxide-decomposible olefin plastic: and 5 to 100 parts of at least one (c) peroxide non-curable hydrocarbon rubber.
Other TPO compositions have been prepared utilizing a thermoplastic polyolefin resin in conjunction with a rubber. U.S. Patent 4,104,210 discloses compositions wherein the rubber is a diolefin rubber, e.g., natural rubber, polyisoprene, nitrile rubber or styrene butadiene rubber. The rubber is fully cured. U.S. Pat. No. 4,211,049 discloses particular curatives for the TPO compositions containing diolefin rubbers, e.g., phenolic curatives, urethane curatives and certain sulfur donor curatives. U.S. Patent 4,141,878 discloses TPO compositions wherein the rubber is cross-linked chlorosulfonated polyethylene.
The TPO compositions of the prior art are either compositions in which the rubber component is uncured or partially cured and have low tensile strength and high compression set or are compositions containing a fully cured rubber portion and consequently are high in hardness and have poor flow characteristics when injection molded, resulting in flow lines in the finished product.